This invention relates to electric induction motors and, more particularly, to induction motors of the axial air gap type.
Induction motors of the axial air gap type have overcome some disadvantages associated with the conventional "cylindrical" type of induction motor. For example, the rotor of an axial gap motor can be entirely supported by the load to be driven by the motor, so that the motor itself need not contain the rotor bearings and associated parts required in a conventional cylindrical machine. However, axial air gap motors have not been commercially successful in any large way and this is believed to be at least in part due to the fact that such motors heretofore designed have incorporated a large number of the disadvantages and restrictive features of the conventional cylindrical machines. For example, some known designs relate to a combined special-purpose motor and pump or compressor unit. The stator component, on the one hand, and the rotor and pump or compressor components, on the other hand, are mounted in separate, mating housing parts, which are assembled together with a sealing member sandwiched between them. Therefore, because part of the motor is a totally integral part of the load device and because the two housing parts are designed and manufactured so as to mate together to form a unitary housing assembly, the motor is not suitable for general purpose use, as is the conventional type of motor. Also, because the whole motor is totally enclosed in the housing parts, cooling of the motor becomes a problem.
A particular problem resides in the fact that these known combination units have to be dismantled by skilled electricians for the purpose of maintenance and repair, e.g. should the stator winding burn out. While this may not pose an insuperable difficulty to the large-scale user in an industrial country (i.e. assuming he has an adequate supply of skilled labour), to a small user, particularly in an undeveloped country, skilled help may not be available and therefore the need to carry out an unexpected repair or maintenance operation may be a tremendous nuisance, particularly when the motor is being used in a continuous processing application and more particularly when the process is of a short-term, high-volume seasonal nature, e.g. the processing of agricultural products.
Another disadvantage of known induction motors, whether of the conventional type or of the axial air gap type, resides in the fact that their operating speeds are not variable, or at least not variable without the provision of expensive and complex ancilliary equipment. In some applications, it is highly desirable that the speed of operation of a motor be changed from time to time, e.g. to change the operating speed of an item of plant in accordance with the nature of a product to be treated or to entirely up-rate a plant. In the former case, it is generally necessary to employ a variable speed transmission, which involves increased cost and reduced efficiency and reliability; in the latter case, the motor may have to be entirely replaced.
It is an object of the present invention, as viewed from one aspect, to provide an electric induction drive assembly in which maintenance is not required; in which inspection and cleaning can be carried out by virtually unskilled labour; which can be modified to run at a different speed by virtually unskilled labour; and which can preferably be installed by virtually unskilled labour.
According to this aspect of the present invention, an electric induction drive assembly comprises a rotor formed from an annular core of magnetic material, radially inner and outer shorting rings between which the core extends and a plurality of rotor bars each extending generally radially between the inner and outer shorting rings, the rotor being coaxially attached to or attachable to a shaft of a load to be driven by the drive assembly, a stator comprising an annular core of magnetic material having windings arranged thereon to produce a magnetic field extending axially from one face of the core, a structure supporting the stator so that, by appropriate positioning of the structure with respect to the load shaft, the drive assembly may be made operable by positioning the stator core with said face thereof confronting one face of the rotor core and spaced by a predetermined amount from said face of the rotor core, the stator and the support structure, on the other hand, being formed as entirely discrete elements, and means releasably securing the stator core in a desired position with respect to the support structure and in such a manner that the stator core and the windings thereon can be removed from and replaced on the support structure when the assembly is arranged in the operable condition without movement of the support structure.
As those skilled in the art will know, when a load is to be driven by an electric motor, so called "half-couplings" are secured to both the load input shaft and the motor output shaft and, after very careful positioning of the motor so that the axes of the two shafts are precisely aligned, the two half-couplings are bolted together. The rotor of the present drive assembly is thus desirably so formed as to be simply securable to the load shaft in the same manner as a conventional half-coupling, whereby it can be fitted to the shaft, literally in moments, by anyone capable of using a screwdriver, wrench or Allen key. The rotor, which is the planar equivalent of the cylindrical squirrel-cage rotor used in conventional cylindrical induction motors, is for all practical purposes electrically indestructible and therefore, once installed, requires no attention. The stator and support structure are fitted in place, either before or after the rotor is fitted, as the case may be, as is described in more detail below.
Provided the rotor will fit on to the shaft and provided the rating of the drive assembly is adequate for the load, a particular drive assembly can be used in a variety of applications and can thus be ordered from the manufacturer on an "off the shelf" basis.
As was mentioned before, the rotor is virtually electrically indestructible and thus the only parts which can fail electrically are the stator windings. As the stator core together with the windings thereon is removable from the support structure without disturbance of the latter, should the stator windings burn out the core can be simply removed by unskilled personnel and replaced by anew one: no casing has to be opened and dismantled to allow access to the stator. In the same way, an undamaged stator can be removed for inspection and/or cleaning and then replaced. The services of a skilled electrician or engineer are never normally required.
The simple replaceability of the stator core and windings gives rise to another important advantage. The speed of the motor can be quickly changed by removing the stator and replacing it by another stator of the same size, but wound so as to have a different number of poles and therefore a different synchronous speed. In a preferred form of the invention, the support structure comprises an open-ended cylindrical frame arranged coaxially of the stator core and the means releasably securing the stator core comprise a plurality of threaded members extending radially inwardly through apertures in the frame. The core and windings are removed in a very straightforward manner by simply unscrewing the threaded members and withdrawing the core and windings from the frame. The same core or a replacement core can then be replaced in converse manner. If a replacement core is at hand, the whole withdrawal and replacement operation takes only a few minutes.
The means releasably securing the stator core may further comprise a plate which fits within the frame and is secured in place by the threaded members which are screwed into tapped holes in the plate, the stator being secured to the plate with said face thereof remote from the plate. With this arrangement, due to the fact that the plate fits within the frame, the location of the threaded members when replacing the core is simplified. The stator core may be secured to the plate by a plurality of axially-directed screws, which extend through apertures in the plate and into tapped holes in the core. Thus, when the plate has been withdrawn, the core and windings may be simply and quickly unscrewed from the plate and a replacement fitted to the plate, which is then reaffixed to the frame.
In an alternative arrangement, the plate is not provided, the threaded members being screwed directly into tapped holes in the stator core. This arrangement has the merits of cheapness and simplicity and also allows good circulation of air between the core and the frame. This arrangement is preferred for convenience to the plate mounting arrangement when, as described below, the stator has windings on both faces.
The support structure may include a flat base member secured to the cylindrical frame and lying in a plane parallel to the axis of the frame. Installation of the assembly is thereby greatly facilitated: the support structure, resting on the base member, is moved about until the rotor and stator are coaxial and have the required nominal air gap between their confronting faces. As the transmission of torque between the stator and rotor is accomplished entirely electromagnetically rather than mechanically, the rotor being supported solely by the load shaft, the alignment step is by no means the skilled precision operation necessary when installing a conventional cylindrical induction motor. In fact, somewhat surprisingly in view of the fact that in prior axial air gap machines pains have been taken to precisely align the rotor and stator and to accurately determine the spacing between them by fixing them in interfitting housing parts, it has been found that the present drive assembly will operate satisfactorily without any significant loss of efficiency or any excessive vibration if the axes are slightly inclined or mutually spaced and/or if the air gap departs significantly from the nominal value. This being so, the positioning of the stator support structure can be carried out by unskilled personnel. The structure merely has to be moved until the rotor and stator are more or less coaxial and the average air gap thickness as measured around the periphery is within about 10% of the nominal value, which varies upwardly from 2.5 mm for a 1 horse-power motor. The base member preferably has apertures therein for receiving fixing members (e.g. bolts), whereby the support structure can be fixed in place once it has been correctly positioned.
To cater for an alternative way of fixing the support structure, the cylindrical frame may have attached thereto, instead of or as well as the base member, a plurality of axially-extending sockets for receiving fixing members, whereby the structure can be secured to a surface lying in a plane perpendicular to the axis of the frame. In this way, the frame can be attached, for example, to the casing of the load or to a framework associated with or forming part of the load. In this case, it may be convenient for the load shaft to extend through the centre of the stator so that the rotor is fitted outside the stator, although the rotor can be fitted inside the stator if there is room, e.g. if the frame is fitted around the lip of a recess.
A phenomenon encountered with known axial air gap motors which can sometimes give rise to problems resides in the axial thrust imparted to the rotor, in use, by electromagnetic action between the rotor and the stator. As an axial air gap induction motor is run up to operating speed from standstill, the rotor and stator first repel one another. The repulsive force decreases as the speed increases and falls to zero at about 45% of synchronous speed. Above this speed value and more particularly at the operating speed, the force becomes one of attraction, the force varying in accordance with the stator current due to its electromagnetic nature.
If the load shaft is mounted in a good thrust bearing, the axial thrust presents little or no problem. However, if the load shaft is not provided with a thrust bearing, or if the thrust bearing is worn, the axial thrust may cause the air gap width to be reduced below acceptable limits, and in extreme cases may give rise to rubbing between the rotor and the stator.
It is an object of the invention, as viewed from a second aspect, to provide an electric induction drive assembly in which the axial thrust due to the electromagnetic attraction between the rotor and stator is at least partly counteracted.
In accordance with this aspect of the present invention, an electric drive assembly comprises a rotor having an annular core of magnetic material, a stator comprising an annular core of magnetic material having windings arranged thereon to produce a magnetic field extending axially from one face of the core, means for positioning the rotor and stator so that said face of the stator core confronts one face of the rotor core with the two faces spaced apart by a predetermined amount, a plurality of electromagnets fixed with respect to the stator and energized by the current flowing through the stator windings, and an electrically-conductive non-magnetic part provided on the rotor and confronting said electromagnets, whereby an electrodynamic repulsive force is produced between the rotor and the stator which opposes the electromagnetic attractive force between the rotor and the stator.
Also, in accordance with this second aspect of the invention, in an assembly in accordance with the first aspect set forth above, a plurality of electromagnets energized by the current flowing through the stator windings are fixed relative to the stator and the rotor is provided with an electrically-conductive non-magnetic part which confronts said electromagnets in the operable condition of the apparatus, whereby an electrodynamic repulsive force is produced, in use, between the rotor and stator which opposes the electromagnetic attractive force between the rotor and the stator.
Although in some cases it may be sufficient only partly to counteract the electromagnetic attractive force, the assembly is preferably so designed that the two forces completely nullify one another. As both forces are dependent on the current through the stator windings, they substantially cancel each other out as the current varies with variation in the load torque. Moreover, as both forces are speed dependent, they also substantially cancel each other out as the speed of the motor varies, in use, with the load.
The electromagnets may comprise magnetic yokes having pole pitch traversing parts of the stator windings wound around them. In this way, copper wastage is minimized by using those parts of the windings which otherwise would produce no useful effect. The electrically-conductive non-magnetic part and the electromagnets may be mounted on the outer or on the inner periphery of the rotor, as desired. In the above described arrangement in which the stator core is mounted on a plate which fits within a cylindrical frame, the yokes may extend axially from the part of the plate between the outer periphery of the stator core and the frame. As this space must in any event be provided to accommodate the pole pitch traversing parts of the stator winding, no increase in size of the stator support structure is necessary to accommodate the yokes.
If the electrically-conductive non-magnetic part of the rotor is on the outer periphery of the rotor, it is desirably made integral with the outer shorting ring to enable the two (together with the rotor bars and the inner shorting ring) to be cast together in one operation. In a preferred arrangement, the electrically-conductive non-magnetic part of the rotor is in the form of a continuous ring which extends around the entire outer periphery of the rotor and lies in a plane perpendicular to the axis of the rotor. A continuous ring ensures in a simple manner that the force does not vary over the cycle of rotation, though a continuous force can be maintained even if the electrically-conductive non-magnetic part is discontinuous, by appropriately disposing the electromagnets and possibly by distributing them among the phases, if the drive assembly is polyphase.
Although the parts of the drive assembly of the invention are well cooled as compared with known devices, due to the fact that the drive assembly is not enclosed in a casing, in some cases forced cooling may be needed. In this case, cooling fan blades may be formed integrally with the electrically-conductive non-magnetic part of the rotor and the outer shorting ring.
In accordance with one particular way of carrying the invention into effect, the stator core has windings arranged in slots on both faces thereof, so as to produce magnetic fields extending axially away from both faces of the core, and a second rotor of like construction is provided, the arrangement being such that each face of the stator core confronts a face of a respective one of the rotor cores, the stator windings being arranged so that both rotors are rotated in the same rotational sense. With this arrangement, substantially twice the torque can be obtained from a stator of a given size.
In the stators of conventional cylindrical induction machines, the copper (or other conductive material) forming the pole pitch traversing parts of the windings produces no useful motive effect. The ratio of this non-useful copper to useful copper, i.e. the copper in the core slots, in typically of the order of 1.25:1. The non-useful copper involves a considerable extra expense. In axial air gap motors having annular stator cores, the above ratio is decreased and can be reduced to less than 1:1. However, this still represents a lot of wasted copper. In the above-described twin rotor arrangement, the amount of wasted copper can be greatly reduced. The windings on both faces of the stator core are formed together, the conductors, as they emerge from the outer exit of each slot, being directed substantially axially over the outer periphery of the stator core to an adjacent slot on the other side of the core. In this way, the wasted copper solely traverses the axial width of the stator core, rather than a large portion of the inner and outer peripheries of the core, and the proportion of wasted copper is therefore small. As the ratio of the axial width to the outer diameter of the core can be decreased as the size of the stator core is increased, the saving in copper is greater for larger drive assemblies. The ratio of wasted to useful copper approaches a very small value as the size of the stator core is increased.